https://leetcode.cn/problems/powx-n/
class Solution {
public:
    double myPow(double x, long long n) {
        if(n==0)
            return 1.0;       
        if(n<0)
            x=1/x;    
        double tmp=myPow(x,abs(n)/2);
        return n%2?tmp*tmp*x:tmp*tmp;
    }
};